In medical diagnosis, nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) requires the production of a very strong static primary magnetic field for passage through a patient's body. A gradient magnetic field varying with time is superimposed on the primary field. Additionally, the patient is exposed to RF electromagnetic waves that are varied in time and in particular patterns. Under the influence of the magnetic and RF waves, nuclear spin distributions of atomic nuclei can be observed. This technique permits soft tissue and organs of the body to be examined for abnormalities such as tumors.
In MRI, the magnetic field must typically be a strong field on the order of about one kilogauss or more. Fields in excess of ten kilogauss (one Tesla) are sometimes required. Additionally, the field must be uniform, with non-uniformities of no more than one hundred (100) ppm. In addition, this uniformity must encompass a large portion of the patient's body, preferably with a diametral spherical volume (DSV) on the order of about 0.3 to 0.5 meters.
In the past these strong magnetic fields have been generated using permanent magnets, resistive magnets or superconducting magnets. Permanent magnets are typically the least expensive, require minimal site preparation, and are low cost to maintain due to no need for liquid cryogens. Permanent magnets however, have limited field strength, temporal instabilities, are very heavy, and costly at field strengths above 0.20 Tesla. Resistive magnets are also relatively inexpensive but require an elaborate and costly power and water supply. In addition the strength of resistive magnets is limited, large unwanted fringe fields are often generated, and temporal instabilities exist. Superconducting solenoidal magnets have the advantage of a strong field with high uniformity and good temporal stability. Currently known superconducting solenoidal magnets, however, are expensive to construct and maintain and require elaborate liquid cryogenic support systems. In the construction of medical MRI magnets, two different configurations or embodiments are generally in use. One type of supporting structure is known in the art as an open access structure. Such a structure typically includes opposite parallel magnetic pole faces mounted on opposite parallel support plates. At least one and usually four support columns support the support plates and provide a return path for magnetic flux. Such an open structure is favored by patients because it is open and accessible from four sides. With such a structure, the magnetic flux lines pass generally orthogonally to the longitudinal (i.e head to toe) axis of the patient.
Another type of MRI magnet, rather than being constructed with opposite magnetic pole faces or an open access structure, is similar to a large conventional solenoid. The solenoid structure is generally cylindrical in shape and is helically wound with electrically conducting wire. An electric current conducted through the wire produces lines of flux that run through a central opening of the cylinder and generally parallel to the longitudinal axis of the patient. Such an enclosed solenoid structure is known to give some patients a claustrophobic reaction.
Different types of magnet systems have been proposed for use with each of these structures. In the past, open access structures have typically been constructed with permanent magnets attached to the opposite pole faces. U.S. Pat. No. 4,943,774 to Breneman et al. for instance, discloses such an open access MRI structure that utilizes permanent magnets. The supporting structure is fabricated of a ferromagnetic material such as high quality structural steel.
The enclosed solenoid type structures may be formed with superconducting magnets. Such superconducting magnets must be cooled to a temperature close to absolute zero (-273.degree. C.) in order for the wiring to lose resistance to the flow of electric current. Relatively small diameter wires can thus carry large currents and create high magnetic fields. The superconducting wires are typically wrapped around the outer periphery of the cylindrical structure enclosed in a cryostat vessel. Such an enclosed solenoid type structure may employ a pair of main superconducting coils and one or more auxiliary coils. Iron or other ferromagnetic materials can be mounted in the patient receiving opening, as shims for adjusting the shape of the magnetic field. These measures are required to create a uniform field having lines of flux substantially parallel to one another and extending through the patient's body.
Another type of enclosed structure is disclosed in U.S. Pat. No. 4,766,378 to Danby et al. In one embodiment of the Danby et al patent, parallel opposite pole faces are mounted on opposite parallel circular support plates. A substantially continuous support frame is located between the support plates to provide support for the pole faces and a return path for flux. An enclosed patient receiving space is located between the pole faces formed by openings through the support frame. For generating a magnetic field, superconducting wires are wound around each pole face and enclosed within a cryostatic vessel. The continuous support frame is generally circular in shape and shapes the field to create a uniform magnetic field within the enclosed patient space. This arrangement is similar to an open access MRI magnet in that the lines of flux pass generally orthogonally to the patient's body. A problem with this type of structure however, is that a patient may feel even more confined than with a solenoid type structure. Additionally the structure is large and relatively heavy and may be difficult to locate in a conventional hospital without extensive site modifications. A further disadvantage is that there is no access for additional medical personnel to perform interventional radiology.
The present invention is directed to an open access MRI magnet that uses superconducting magnets to generate a uniform magnitude flux field without the use of a heavy and confining support frame, and without the necessity for supplying a liquid cryogen to the system. In addition, the open access frame can be used to provide access for interventional radiology. Accordingly it is an object of the present invention to provide an open access MRI magnet having a uniform magnetic flux field generated by superconducting magnets which are cooled without the use of liquid cryogenics. It is another object of the invention to provide an open access MRI magnet in which a strong magnetic flux field on the order of 0.20-0.5 Tesla can be generated and shaped in a structure that is open for interventional radiology and is not confining to a patient. It is a further object of the present invention to provide an MRI magnet that is simple and inexpensive to build and to operate.